Switching power supply

Details Switching power supply Switching power supply

2002-03-13

2003-06-17

Patel, Rajnikant B. (Department: 2838)

Electric power conversion systems

Current conversion

With condition responsive means to control the output...

C363S021060, C363S015000

Reexamination Certificate

active

06580626

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a synchronous rectifying switching power supply for turning on or off a rectifying switch element and/or a free-wheeling switch element on the secondary side of a transformer, synchronously with a switching element.
2. Description of the Related Art
In a conventional DC/DC converter in which a DC input voltage is intermittently applied to a primary winding of a transformer through a high-frequency switching action of a main switching element, so that an AC voltage induced in a secondary winding of the transformer is rectified by a rectifying element to thereby obtain a DC output voltage, a circuit topology such that a MOSFET is used as a rectifying element or a free-wheeling element provided on the secondary side of the transformer so that the MOSFETs are turned on or off synchronously with the switching action of the main switching element, has been well recognized by those skilled in the art as an effective means for reducing power loss in the circuit elements. However, if a parallel running is performed by connecting several (two, for example) synchronous rectifying DC/DC converters to a common load, the following problems occur.
That is, if the loads in the respective DC/DC converters were well balanced but an output voltage in a second DC/DC converter rises for some reason such as load change, a first DC/DC converter allows its built-in control circuit to detect such rise in the output voltage to thereby control for lowering the output voltage, i.e., for narrowing a pulse conduction width of the main switching element. If such control reaches a limit, the main switching element stops operating, so that the output voltage is applied from the operating second DC/DC converter to the output circuit of the non-operating first DC/DC converter, and thus the gate of the rectifying MOSFET is forward biased, thereby resulting in the turning on of the MOSFET. Then, the electric current is allowed to flow into the secondary winding of the transformer from the second DC/DC converter through the MOSFET, so that a core of the transformer gets saturated and thus the secondary winding gets into a state of substantial short circuit, which allows further strong current to flow in the MOSFET, thereby occasionally damaging the MOSFET. On the other hand, the first DC/DC converter continues receiving the current from the second DC/DC converter, so that the rectifying and free-wheeling MOSFETs start self-oscillation, thus causing, though it depends on cases, failures in the elements due to the heat generated thereby.
FIG. 9
is a circuit diagram showing a specific example of such conventional parallel running switching power supply. In
FIG. 9
, reference numerals
1
A,
1
B . . . designate DC/DC converters connected in parallel and
3
a DC power source for supplying a DC input voltage Vi to the respective DC/DC converters
1
A,
1
B . . . , wherein the respective DC/DC converters have the same circuit topology. In the respective DC/DC converters
1
A,
1
B . . . , reference numeral
5
designates a transformer of which the primary and the secondary sides are isolated from each other. Reference numeral
8
designates a main switching element such as a MOSFET which is connected in series with the primary winding
6
of the transformer
5
. The main switching element
8
turns on or off so that the DC input voltage Vi is intermittently applied to the primary winding
6
of the transformer
5
so as to take out AC voltage from the secondary winding
7
of the transformer
5
.
Across the primary winding
6
is connected an active clamp circuit
71
comprising a series circuit of an auxiliary switching element
9
including a MOSFET and a capacitor
10
. The main switching element
8
and the auxiliary switching element
9
are turned on or off alternately, defining an off period or dead time, respectively. Thus, the magnetizing inductance of the transformer
5
, parasitic capacitance of the respective switch elements
8
,
9
(see
FIG. 10
) are allowed to resonate, thereby achieving Zero Voltage Switching at the time of the turn-on and turn-off of the switching elements
8
,
9
. In the meantime, reference numeral
72
designates a body diode which is connected in parallel with reverse polarity across the drain and the source of the switching element
8
. Likewise,
73
also a body diode which is connected in parallel with reverse polarity across the drain and the source of the auxiliary switching element
9
.
A MOSFET
11
serving as a rectifying element is connected in series with the secondary winding
7
of the transformer
5
, while a MOSFET
22
serving as a free-wheeling element is connected between the series circuit of the secondary winding
7
and the MOSFET
11
. The gate of the MOSFET
11
is connected to a dotted side terminal of the secondary winding
7
where a positive voltage is induced when the main switching element
8
turns on, while the gate of the MOSFET
22
is connected to a non-dotted side terminal of the secondary winding
7
where a positive voltage is induced when the main switching element
8
turns off. A series circuit of a choke coil
13
and a smoothing capacitor
14
is connected across the MOSFET
22
. By turning on or off the MOSFETs
11
and
12
synchronously with the main switching element
8
, an AC voltage generated in the secondary winding
7
of the transformer
5
is rectified, which is further smoothed by the choke coil
13
and the smoothing capacitor
14
, whereby a DC output voltage Vo can be obtained from both terminals of the smoothing capacitor
14
. In the meantime, reference numerals
75
and
76
designate body diodes each of which is connected in parallel with reverse polarity across the drain and the source of the MOSFETs
11
and
22
.
Reference numeral
17
designates a control circuit for monitoring the DC output voltage Vo and varying a pulse conduction width of a drive signal to be supplied to the gate of the main switching element
8
or the auxiliary switching element
9
, corresponding to the change in the DC output voltage Vo, thereby stabilizing the DC output voltage Vo through the feedback by the control circuit
17
.
FIG. 10
is a circuit diagram of the DC/DC converter
1
A which ceased operating due to the difference in output voltage Vo in the parallel running switching power supply of FIG.
9
. Here, parasitic capacitances
82
to
85
of the respective switching elements
8
,
9
and the MOSFETs
11
and
12
are taken into consideration. Each switching element
8
,
9
on the primary side of the transformer
5
is in a completely off-state. The main switching element
8
is connected to a parallel circuit of the body diode
72
and the parasitic capacitance
82
, while the auxiliary switching element
9
is connected to a parallel circuit of the body diode
73
and the parasitic capacitance
83
, respectively. Further, the second DC/DC converter
1
B for supplying the output voltage Vo, which equivalently serves as a voltage source
87
, is connected to the secondary side of the transformer
5
.
In a state illustrated in
FIG. 10
, the MOSFETs
11
and
12
start self-oscillation, through Stages 1 to 4 shown in waveform diagrams of FIG.
11
. In the waveform diagrams of
FIG. 11
, the uppermost waveform indicates a drain-source voltage VSR1 of the MOSFET
11
, and the next waveform immediate therebelow indicates a drain-source voltage VSR2 of the MOSFET
22
, then a inductor current iL flowing through the choke coil
13
, and an magnetizing current iLm flowing in the secondary winding
7
of the transformer
5
, in sequence.
FIG. 12
shows the equivalent circuit for State
1
. Reference Numeral
91
designates a combined capacitance on the primary side of the transformer
5
. If the capacitance of the respective parasitic capacitances
82
,
83
are denoted by CQ
1
, CQ
2
, while the turn ratio of the primary winding
6
to the secondary winding
7
is n:1, then the composed capacitance equals n
2
(CQ
1
+CQ
2
). Further, reference numera

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